1. Field of the Invention
The present invention relates generally to collapse-resistant, flexible thermoplastic tubing, to such plastic tubing which includes plural helical conductors extending along the length of the tubing, and to a method for making such flexible plastic tubing.
2. Description of Conventional Technology
Tubing having a relatively thin wall and an integral helical supporting bead is known in the art. Such support-bead tubing construction provides substantial crush resistance while leaving the tube wall flexible enough to permit short-radius bends without collapsing or kinking the tube. The versatility of this kind of tubing is evidenced by its wide applicability in construction, ventilation, manufacturing processes, auto washes, hospitals, medical devices, and other fields. In medical applications the necessity to maintain a clean environment dictates that the tubing be free of crevices inside or outside in which soil or bacterial contamination could reside. A smooth inner bore for the tubing is also important because flow resistance must be kept to a minimum when a tidal air flow is conducted through the tubing, as is the case with inhalation therapy and other breathing-assistance machines.
U.S. Pat. No. 3,910,808 to Steward, assigned to the same assignee as this application, discloses apparatus for forming such thin-walled, flexible, crush resistant support-bead tubing.
Steward discloses means for extruding a plastic strip having a longitudinal rib, and winding means for helically winding the strip about an axis to produce a corrugated flexible tubing having a smooth bore.
Further, U.S. Pat. No. 5,454,061, also to Steward, discloses apparatus for forming such thin-walled, flexible, crush resistant support-bead tubing also including a helically disposed conductor. Steward '061 discloses a tubing in which the conductor is disposed adjacent to overlapping side edges of an extruded plastic strip or ribbon, which is wrapped helically on itself while molten to form the tubing wall. The overlapped edges of the ribbon as well as the conductor are then helically covered with an extruded molten bead, which also bonds with and becomes an integral part of the tubing.
With the Steward '061 tubing, and with other conventional tubing, when plural conductors are disposed in the tubing wall, they are immediately adjacent to one another. In some applications when the conductors are used for warming, heating, or maintaining the temperature of a tidal air flow in the tubing by means of resistance heating, for example, the heating from these conductors is concentrated at their location. It may be desirable for the conductors to not be immediately adjacent to one another so that heating is somewhat more dispersed. Further, when plural conductors are disposed in such a tubing wall, and one or more of the conductors is utilized for conducting instrumentation or telemetry signals, then it is also desirable for the conductors to be spaced apart a controlled and determined distance for purposes of preserving signal clarity and fidelity, for insuring electrical insulation between the conductors, and for preventing or reducing mutual inductive coupling among the heating and among the signal conductors.
No prior product, method of manufacture, or apparatus is known which provides a thin-walled, smooth-bore tube having plural spaced apart conductors helically imbedded in the outer structure of the tube and which conductors are available for heating of tidal air flow in the tube, as well as for instrumentation or telemetry signals to be conducted along the length of the tube, for example, all with a helically ribbed but substantially crevice-free outer surface.